Often the fairing falls off or blisters are found, this is caused by contamination between the lead and the fairing. I had this problem and found sand from the mold
that was used to cast the keel.
To fix this, you need to get epoxy
to stick to lead. You do that by coating the lead with an un-thickened epoxy
and then using a steel
brush to vigorously brush the fresh (right after coating) epoxy into the lead. Do the fairing as soon as this coat has gelled but is still sticky.
I didn't do this the first time so I had to do it again next haul out